Mr Lincoln, a former BBC scriptwriter for Z Cars and Dr Who, said that the thriller, which has sold over 30 million copies, making it one of the all time best-sellers, was "purely a fantasy thriller which has nothing to do with anything".

The writer, now in his seventies, was the first to think of writing The Holy Blood and The Holy Grail, which sold 2 million copies in the 1980s with its modern quest for the Holy Grail, suggesting that the Grail was not the chalice that Christ drank from at the Last Supper, but a bloodline, from his possible marriage to Mary Magdalene, secretly protected by the Knights Templar.

He wrote the book with Michael Baigent, a New Zealand journalist who moved to England in 1976, and Richard Leigh, a novelist from New Jersey, who settled in London in 1974.

Mr Lincoln has steered clear of a lawsuit brought by his co-authors against Brown, who, they claim, lifted the "complete architecture" of the book, including the central hypothesis that Christ married and produced a child.

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If they are successful and opt to take injunctions stopping the use of their material, they could threaten the Cannes premiere next month of Hollywood's adaptation of the novel, starring Tom Hanks, Sir Ian McKellen and Audrey Tatou.

Brown insists that he read The Holy Blood and The Holy Grail long after writing the synopsis of his novel. His publishers, Random House, who are being sued for copyright infringement, could face a claim for millions of pounds of damages. The decision will clarify existing copyright laws over the extent to which an author can use other people's research.

Until now Mr Lincoln, of Todenham, Glos, has remained virtually silent about the case. Last week he said that inference could be drawn from comments by the judge. This was thought to refer to reports that he was unwilling to run the risk, and cost, of losing the case.

However, speaking on BBC Radio 4, Mr Lincoln backed his fellow authors, saying that his book reflected ideas that were in circulation, which "I neither believe not disbelieve".